Coordinate measuring machines are known of the type including a bridge supported on a base for guided movement along a first horizontal axis, a carriage mounted on the bridge for guided movement along a second horizontal axis, and a probe mounted for guided vertical movement on the carriage. Thus, guided movement of the probe tip is provided along three orthogonal axes with respect to the horizontal surface of a table on which an object to be measured is supported.
In such machines, transducer means are provided to generate electrical signals precisely corresponding to the probe movement along each axis. Movement of the probe between points on the object thus allows precision measurement between these points on the object by appropriate processing and display of the transducer signals.
In these machines, the bridge is mounted to be movable along "Y" axis ways formed on the base, the bridge having a pair of upstanding members extending above the table surface, with a connecting cross member extending thereacross, and supporting the carriage for movement along "X" axis ways formed on the crossmember. The probe is mounted for vertical movement on the carriage along a "Z"-axis.
In U.S. Pat. No. 4,594,791, issued June 17, 1986, for "A Bridge Type Coordinate Measuring Machine", there is described a "ring bridge" construction in which the bridge completely encircles the table to form a "closed loop" and thereby improve the rigidity of the bridge.
Rigidity is critical so as to insure precise repeatable correspondence between the movement of the probe and the measured movement of the bridge and carriage along each of the X and Y axes.
In order to improve the speed with which the bridge may be driven, a lightweight construction of aluminum is employed. In order that differences in thermal growth characteristics do not create slight variations of the bridge movement on the ways when temperature fluctuations occur, the base must also be constructed of aluminum.
On the other hand, the table, which directly supports the object to be measured, should be quite rigid in order to not be deflected excessively from the weight of heavy objects to be measured, such as engine blocks and the like. Steel is thus a desirable material from which to construct the table so as to provide a rigid surface with reasonable table dimensions. The use of a diverse material in the table makes desirable an arrangement for mounting the table to the base to minimize distortions of the base which could effect the movement of the bridge thereon by temperature changes occurring during the making of measurements.
The weight of the table and object should itself not cause excessive distortion of the base and the ways, particularly with heavier measured objects.
Additionally, access by the probe to all points on the table is desirable in order that the entire surface of the table provides a useful operating area of the machine. If the bridge movement is not adequate to enable such probe access, the table surface available for making measurements is reduced, to lessen the capacity of the machine.